1. Field of the Invention
The present invention relates to an active optical fibre doped with rare earth elements. Furthermore, the present invention relates to an optical amplifier comprising said active optical fibre, an optical communication system comprising said optical amplifier, and a method for producing said optical fibre.
2. Technical Background
As known, at present optical fibres are widely used in the field of telecommunications for transmitting signals. They essentially comprise an inner cylindrical region, called core, within which a signal is transmitted, and an outer annular region, called cladding. The refractive index of the cladding is lower than that of the core, so as to confine the transmitted signal within the latter.
Typically, both the core and the cladding are made of a silica-based glass material. The difference of refractive index between the core and the cladding is obtained by incorporating suitable additives (dopants) into the glass matrix of the core and/or of the cladding.
Typical examples of dopants used for modifying the refractive index of silica are germanium and phosphorous (which increase its refractive index) and fluorine (which decreases its refractive index).
An active optical fibre is an optical fibre whose core is further doped with particular substances capable of giving effects of optical amplification.
Typical examples of said substances are rare earth ions, whose spectroscopic properties are particularly suitable for the purpose. Among rare earths, erbium is the most frequently used component since its fluorescence spectrum has a band ranging between 1420 and 1650 nm, which corresponds to the third transmission window (centred at about 1550 nm) of a telecommunication signal.
Active optical fibres are used for producing optical amplifiers or, for example, super-fluorescent sources or lasers.
In general, an optical amplifier comprises an active optical fibre and a luminous source, called xe2x80x9cpumping sourcexe2x80x9d, suitable to provide a pumping signal having a wavelength (typically corresponding to a peak of the absorption spectrum of the dopant substance) capable of bringing the ions of the dopant substance to an excited energetic level. From said level, the ions spontaneously fallxe2x80x94in very short timesxe2x80x94to a laser-emission level, or metastable level, where they remain for a relatively longer time (called mean lifetime of the metastable level).
When a luminous signal having a wavelength corresponding to said metastable level passes through an active optical fibre having a high number of excited ions on the metastable level, the excited ions decade to a lower level, thus causing a stimulated luminous emission having the same wavelength as the signal.
High concentrations of rare earth ions inside the core of an active optical fibre reduce the length of fibre needed for obtaining a high amplification of the signal.
Nevertheless, due to the extremely compact structure of silica, inside the core of an active optical fibre the rare earth ions tend to aggregate with one another as their concentration inside the core increases (this phenomenon is conventionally known as xe2x80x9cclusteringxe2x80x9d).
Sincexe2x80x94when aggregatexe2x80x94excited rare earth ions tend to fall from the metastable level in a non-radiative way, said aggregation causes a reduction of mean lifetime of the excited ions on the metastable level and thus, of the efficiency of fluorescence of the active optical fibre.
As the clustering prevents a corresponding increase of the fluorescence efficiency when the concentration of rare earth ions inside the core of an active fibre increases, it actually limits the value of maximum concentration of rare earth ions inside the core of an active optical fibre.
In the present invention and claims, the expression
xe2x80x9cfluorescence efficiencyxe2x80x9d of a material indicates the ratio between the power of the amplified spontaneous emission (ASE) back-diffused by the material, in absence of transmission signal and during absorption of electromagnetic radiations by a pump source, and the power of the electromagnetic radiations received from said source (pumping power); and
xe2x80x9camplification efficiencyxe2x80x9d of an optical amplifier indicates the ratio between the optical power of a transmission signal in output from it and the optical pumping power provided to it.
The amplification efficiency of an optical amplifier increases as the fluorescence efficiency of the rare earth with which the active optical fibre is doped increases.
B. James Ainslie (xe2x80x9cA review of the fabrication and properties of erbium-doped fibers for optical amplifiersxe2x80x9d, Journal of Lightwave Technology, 1991, vol. 9, no. 2, pages 220-227) presents a review of methods of fabrication of active optical fibres doped with erbium and of the properties of said fibres. Among the other things, the Author states that the addition of P2O5 and Al2O3 to a silica-based glass enables the incorporation of several weight percents of rare earth ions without clustering effects. Furthermore, studies carried out on the concentration of erbium ions into Al2O3xe2x80x94SiO2 and GeO2xe2x80x94SiO2 have shown that the weight percents of erbium ions that can be incorporated without clustering effects into silica- and alumina-based glasses (Al2O3xe2x80x94Si2) is greater than the weight percents that can be incorporated into silica- and germanium-based glasses (GeO2xe2x80x94SiO2).
Nevertheless, the Applicant found that alumina has a limited solubility into the silica-based glass of conventional optical fibres (typically, the maximum quantity of alumina is less than or equal to, about 11%).
The Japanese patent application JP 3235923 describes an optical amplifier with an amplifying optical fibre suitable to amplify at the same time both signals in the band of 1300 nm and signals in the band of 1550 nm. Said amplifying optical fibre has both the core and the cladding made up of at least one glass material selected among a CaOxe2x80x94Al2O3xe2x80x94SiO2 based glass, a phosphate-based glass and a fluorophosphate-based glass. Moreover, at least one between erbium and neodymium is included in at least one between the core and the cladding.
Nevertheless, the Applicant points out that said document does not give any further information on the composition of the CaOxe2x80x94Al2O3xe2x80x94SiO2 based glass. In addition, as the active optical fibre it describes has both the core and the cladding made up of at least one glass material selected among a CaOxe2x80x94Al2O3xe2x80x94SiO2 based glass, a phosphate-based glass and a fluorophosphate-based glass, it presents difficulties of junction with the optical fibres conventionally used for telecommunications, having a pure silica cladding.
For the production of an active optical fibre, the glasses of the core and cladding with different composition must be compatible with one another. For example, said glasses must be compatible in terms of temperature of glass transition.
This is an important requirement because during some steps of the production process of an optical fibre, the glass materials of core and cladding must both be at a plastic state (neither having a too high viscosity nor being at a melted state). This is only possible if the two materials have such temperatures of glass transition as to guarantee at least a partial superimposition of the temperature ranges in which they are at a plastic state.
The Applicant faced the technical problem of increasing the concentration of rare earth ions inside the core of an active optical fibre, limiting at the same time the ion-clustering phenomenon so as to obtain a corresponding increase in the fluorescence efficiency.
The Applicant has found that said problem can be solved with an active optical fibre having a silica glass cladding and a core comprising a quantity of silica (SiO2) of at least 50% in weight, and a quantity of an oxide XO not exceeding 40% in weight, wherein X is selected from the group comprising Ca, Sr, Ba and Zn.
The Applicant has proved that the core of the active optical fibre comprising, according to the invention, the above percentages of SiO2 and XO is compatible with the silica glass of the cladding in terms of temperature of glass transition (that is, the temperature range in which it is at the plastic state is at least partly superimposable to that of the cladding glass).
Moreover, among the above elements Ca, Sr, Ba and Zn, it has been proved that calcium is preferable since it has a greater solubility in the glass matrix of silica.
Thus, in a first aspect thereof, the present invention relates to an active optical fibre having:
a silica glass cladding, and
a glass core, doped with a rare earth, comprising a quantity of SiO2 of at least 50% in weight, characterised in that the core also comprises a quantity of an oxide XO not exceeding 40% in weight, where X is selected from the group comprising Ca, Sr, Ba and Zn.
The Applicant has experimentally proved that the glass core of the active optical fibre of the present invention has a reduced clustering, thus advantageously allowing the increase of both the concentration of rare earth ions in the core, and the fluorescence efficiency obtainable with said concentration values.
Moreover, the Applicant has proved that the glass core of the active optical fibre according to the present invention xe2x80x94comprising a quantity of silica (SiO2) of at least 50% in weight and a quantity of oxide XO not exceeding 40% in weightxe2x80x94has a greater refractive index than that of the cladding. It thus allows meeting the requirements for the transmission of an optical signal in an optical fibre.
Preferably, said element X is calcium. In fact, as already said above, it has been proved that it has a greater solubility in the glass matrix of the silica.
Preferably, the quantity of SiO2 in the core is in the range from 60 to 90% in weight. More preferably, it is in the range from 70 to 90% in weight. Said values advantageously allow improving the compatibility in terms of temperature of glass transition between the glass of the core and that of the cladding.
Typically, the core also comprises a quantity of germanium dioxide (GeO2) not exceeding 10% in weight.
According to a preferred embodiment, the core also comprises a quantity of alumina (Al2O3) not exceeding 25% in weight.
This preferred embodiment is advantageous because as alumina has a high crystalline field, it has the capacity of perturbing the orbital f of the rare earth, screened by the outer valence orbital, broadening and flattening the fluorescence spectrum of the rare earth.
This allows making the active optical fibre suitable to be used in WDM optical communication systems, since the flatter and broader the fluorescence spectrum of the rare earth, the greater the number of signals at different wavelength that can be evenly amplified.
In the present description and claims, the expression
* xe2x80x9ccrystalline fieldxe2x80x9d is used to indicate the mean potential energy of interaction between two charged particles (for example, two ions). More in particular, the crystalline field J is expressed by the following relation (P. W. Atkins, xe2x80x9cMolecular Quantum Mechanicsxe2x80x9d, Oxford University Press, 1984, Second Edition, page 225):   J  =                    e        2                    4        ⁢                  πϵ          0                      ⁢          ∫                                    "LeftBracketingBar"                                          Ψ                                                      n                    ⁢                                          xe2x80x83                                        ⁢                    1                                    ,                                      /                    1                                    ,                                      m                    ⁢                                          xe2x80x83                                        ⁢                    1                                                              ⁡                              (                                  r                  1                                )                                      "RightBracketingBar"                    2                ⁢                  (                      1                          r                              1                ,                2                                              )                ⁢                              "LeftBracketingBar"                                          Ψ                                                      n                    ⁢                                          xe2x80x83                                        ⁢                    2                                    ,                                      /                    2                                    ,                                      m                    ⁢                                          xe2x80x83                                        ⁢                    2                                                              ⁡                              (                                  r                  2                                )                                      "RightBracketingBar"                    2                ⁢                  ⅆ                      τ            1                          ⁢                  τ          2                    
wherein
xe2x80x9cexe2x80x9d is the electric charge associated to an electron;
∈0 is the dielectric constant in vacuum;
xcexa8 (r) is the wave function of a charged particle;
r1, r2 are position vectors;
r1,2 is the distance from the first charged particle to the second one;
xcfx841, xcfx842 are variables of integration in space;
and the expression
* xe2x80x9celement at high crystalline fieldxe2x80x9d is used to indicate an element having a crystalline field greater than about 0.8xc3x9710xe2x88x9217.
The Applicant has proved that the glass core of the active optical fibre comprising, according to this preferred embodiment of the invention, a quantity of alumina not exceeding 25% in weight, is compatible with the silica glass of the cladding in terms of glass transition.
In the presence of alumina, the quantity of oxide XO is advantageously in the range from 20 to 60% in weight of the total weight of alumina. Preferably, it is in the range from 25 to 45% in weight of the total weight of alumina.
It has been proved that said quantities of oxide XO with respect to the total weight of alumina allow obtaining a good compromise between the phenomenon of broadening and flattening of the fluorescence spectrum of the rare earth due to alumina, and that of reduction of the clustering due to the oxide XO. In fact, too high quantities of XO can limit the broadening and the flattening of the fluorescence spectrum of the rare earth due to alumina, whereas too low quantities of XO can limit the reduction of clustering due to the oxide XO itself.
Moreover, it has been proved that said quantities of oxide XO allow the glass core of the optical fibre of the invention to be compatible with the silica glass of the cladding in terms of temperature of glass transition.
Advantageously, besides alumina, the core also comprises a predetermined quantity of a compound YOz, where z is equal to 1 or 2, and Y is an element having a high crystalline field.
This is advantageous because the element Y with a high crystalline field has the capacity, like alumina, of interacting with the energy levels of the rare earth ions corresponding to the orbitals f, broadening and flattening the fluorescence spectrum of the rare earth.
Preferably, said quantity of said compound YOz does not exceed 7% in weight. Said quantity allows preserving the glass characteristics of the material of the core.
In this case, the quantity of oxide XO is advantageously in the range from 20 to 60% in weight of the total weight of alumina and of the compound YOz. Preferably, the quantity of oxide XO is in the range from 25 to 45% in weight of the total weight of alumina and of the compound YOz.
It has been proved that the above quantities of alumina, of said compound YOz and of the oxide XO allow the glass core of the optical fibre of the invention to be compatible with the silica glass of the cladding in terms of temperature of glass transition.
Typically, said compound YOz is selected from the group comprising ZrO2, BeO and ZnO.
Advantageously, the core has a concentration of rare earth ions up to 1020 ions/cm3.
The Applicant has found that, although the core of the active optical fibre according to the invention allows exceeding these values, greater concentrations of rare earth ions can be disadvantageous because as the mean distance between ions decreases, they can cause an undesired phononic relaxation (decay of the ions from the metastable level in a non-radiative way).
Advantageously, said rare earth in the glass of the core has a fluorescence efficiency of at least 50%. Preferably, said efficiency is of at least 54%.
Advantageously, said rare earth is erbium.
Preferably, said active optical fibre has a numerical aperture (NA) of at least 0.25. More preferably, said numerical aperture (NA) is of at least 0.27. High values of numerical aperture are advantageous since the fluorescence efficiency increases as the numerical aperture increases.
Advantageously, the silica glass cladding comprises a quantity of silica of at least 90%. Preferably, said quantity of silica is of at least 95%.
Moreover, in the proximity of the core-cladding interface, the active optical fibre advantageously comprises an annular layer of silica glass doped with a dopant that modifies its thermal expansion coefficient so as to make the thermal expansion coefficient of the cladding compatible with that of the core.
In fact, as the glasses of the core and of the cladding have a different composition, they also have different coefficients of thermal expansion. This may also bring to manufacture problems, as discussed in U.S. Pat. No. 4,339,173, which refers to fibres for transmitting luminous signals and describes the addition of B2O3 to the cladding of such fibre to reduce the differences in the coefficient of thermal expansion of the core and of the cladding, thus preventing the formation of cracks at the core-cladding interface of the preform during the cooling.
Moreover, as described in EP 0 602 467 filed by the same Applicant, the difference of the coefficients of thermal expansion causes stresses in correspondence with the core-cladding interface, and a background attenuation connected to said stresses.
Advantageously, said dopant that modifies the coefficient of thermal expansion of said annular layer is at least one among P2O5, GeO2 and B2O3, present in a quantity that increases from the radially outer portion to the radially inner portion of said annular layer.
Moreover, since said dopant modifier of coefficient of thermal expansion of the annular layer can also modify its refractive index, said annular layer advantageously comprises also a dopant modifier of refractive index present in a quantity that increases from its radially outer portion to its radially inner portion.
This allows compensating the change of the refractive index caused by the dopant modifier of the coefficient of thermal expansion so that the refractive index of said annular layer is substantially constant for all of its thickness and substantially equal to, or less than, that of the cladding.
A typical example of this latter dopant modifier of refractive index is F2, when the dopant modifier of the coefficient of thermal expansion raises the refractive index.
In a second aspect thereof, the present invention also relates to an optical amplifier for amplifying an optical signal having a signal wavelength xcexs, comprising
an active optical fibre with a silica glass cladding and a glass core, doped with a rare earth, comprising a quantity of SiO2 of at least 50% in weight;
a pumping source for providing a pumping light at a predetermined pumping wavelength xcexp;
an optical coupler for coupling said optical signal to be amplified and said pumping light into said active optical fibre;
characterised in that the core also comprises a quantity of an oxide XO not exceeding 40% in weight, where X is selected from the group comprising Ca, Sr, Ba and Zn.
Preferably, said rare earth is erbium.
Advantageously, said pumping wavelength xcexp is equal to about 980 nm or 1480 nm.
In a preferred embodiment, said optical amplifier has an amplification band comprised between 1520 and 1630. Preferably, said band is comprised between 1570 and 1620 nm.
Optical amplifiers with an amplification band around about 1570-1600 nm are, for example, described by Hirotaka Ono et al. (xe2x80x9cGain-Flattened Er3+ doped fiber amplifier for a WDM signal in the 1.57-1.60 xcexcm wavelength regionxe2x80x9d, IEEE Photonics Technology Letters, vol. 9, no. 5, May 1997, pages 596-598); H. Masuda et al. (xe2x80x9cWideband, gain-flattened, erbium-doped fibre amplifiers with 3 dB bandwidths of  greater than 50 nmxe2x80x9d, Electronics Letters, vol. 33, no. 12, June 1997, pages 1070-1072) and in the U.S. Pat. No. 5,500,764.
As regards the functional and structural characteristics of said active optical fibre, reference shall be made to what described above relating to the first aspect of the invention.
In a third aspect thereof, the present invention relates to an optical communication system comprising
a transmitting station suitable to provide an optical signal having a signal wavelength xcexs;
an optical transmission line optically connected to said transmitting station, for transmitting said optical signal;
a receiving station optically connected to said optical transmission line for receiving said optical signal;
at least one optical amplifier for amplifying said optical signal, in turn comprising
an active optical fibre with a silica glass cladding and a glass core, doped with a rare earth, comprising a quantity of SiO2 of at least 50% in weight;
a pumping source for providing a pumping light at a predetermined pumping wavelength xcexp,
an optical coupler for coupling said optical signal and said pumping light into said active optical fibre;
characterised in that the core also comprises a quantity of an oxide XO not exceeding 40% in weight, where X is selected from the group comprising Ca, Sr, Ba and Zn.
As regards the functional and structural characteristics of said active optical fibre and of said optical amplifier, reference shall be made to what described above relating to the first and to the second aspect of the invention.
Advantageously, said optical signal is a WDM optical signal comprising a plurality of N signals having wavelengths xcex1, xcex2, . . . , xcexN.
In a fourth aspect thereof, the present invention also relates to a method for producing an active optical fibre, having a core and a cladding, comprising the steps of
a) arranging a tubular support of silica glass;
b) laying inside said tubular support a glass powder comprising a quantity of SiO2 of at least 50% in weight;
c) immersing the glass powder in a solution comprising a solvent and a salt of a rare earth;
d) making the solvent evaporate;
e) heating the tubular support so as to sinter the glass powder;
f) making the tubular support collapse so as to obtain a preform;
g) drawing said preform so as to obtain the active optical fibre;
characterised in that at step c), said solution also comprises a salt of an element X2+, where X is selected from the group comprising Ca, Sr, Ba and Zn, in such quantity as to obtain a quantity of an oxide XO not exceeding 40% in weight in the core of the active optical fibre.
Typical examples of rare earth salts are, in the case of erbium, ErCl3 and ErI3.
Preferably, said element X is calcium.
Typical examples of salts of X2+ are, in the case of calcium, CaCO3, CaCl2 and Ca(NO3)2.
Typical examples of solvent for step c) are methanol and water.
Advantageously, in step c) said solution also comprises a salt of Al3+ in such quantity as to obtain a quantity of Al2O3 not exceeding 25% in weight in the core of the active optical fibre.
Typical examples of salts of Al3+ are AlCl3 and Al(NO3)3.
In this case, in the solution of said step c) the quantity of the salt of the element X2+ is advantageously such as to obtain a quantity of the oxide XO in the range from 20 to 60% in weight of the total weight of Al2O3 in the core of the active optical fibre. Preferably, the quantity of the salt of the element X2xe2x88x92 is such as to obtain a quantity of the oxide XO in the range from 25 to 45% in weight of the total weight of Al2O3 in the core of the active optical fibre.
Advantageously, at step c), besides the salt of Al3+, said solution also comprises a salt of another element Y at a high crystalline field.
In this case, in the solution of said step c) the quantity of the salt of the element X2+ is advantageously such as to obtain a quantity of the oxide XO in the range from 20 to 60% in weight of the total weight of Al2O3 and of a compound YOz (with z equal to 1 or 2) in the core of the active optical fibre.
Preferably, in the solution of step c), the quantity of salt of said element Y at high crystalline field is such as to obtain a quantity of said compound YOz not exceeding 7% in weight in the core of the active optical fibre.
Typically, said compound YOz is selected from the group comprising ZrO2, BeO and ZnO.
Typically, in step b) said glass powder also comprises a quantity of GeO2 not exceeding 10% in weight.
Optionally, between step a) and step b), the method of the invention also comprises the step of laying a number of glass layersxe2x80x94solidified starting from the reactants SiCl4, POCl3, SF6 and/or GeCl4 in vapour phasexe2x80x94into the tubular support in order to form an annular layer at the core-cladding interface suitable to make the core and the cladding compatible in terms of coefficient of thermal expansion.